In recent years, global warming due to an increase of atmospheric carbon dioxide has come to be regarded as a problem. Accordingly, there have been attempts to capture the carbon dioxide emissions from coal-fired power plants and industrial smokestacks and sequester it by burying it deep within the earth or the oceans.
Carbonaceous fuels such as coal, oil, natural gas, biomass or petroleum coke are abundant and low in cost and can be used for power generation. Different technologies for power generation are available on the market. Conventional power plant technologies such as Pulverized Coal (PC) or Natural Gas Combined Cycle (NGCC) typically incinerate the fossil fuel by the use of air, oxygen enriched air or oxygen. Triggered by stricter environmental regulations, the search for a power generation process with higher efficiencies and an increasing demand for using petroleum coke and biomass as feedstock, integrated gasification combined cycle (IGCC) systems have been developed which have the potential to achieve improved efficiencies in comparison to conventional power plant. In such a system, syngas (a mixture of hydrogen and carbon monoxide) is produced by partial oxidation of coal or other carbonaceous fuel. This allows cleanup of sulfur and other impurities before combustion. Moreover, if carbon sequestration is desired, the carbon monoxide can be reacted with steam using the water gas shift reaction to form carbon dioxide and hydrogen. Carbon dioxide may then be recovered using conventional technologies known in the art. This allows pre-combustion recovery of carbon dioxide for sequestration.
WO 2008/157433 describes a hybrid IGCC plant which is modified to provide carbon capture and storage, in which the syngas leaving the warm gas cleanup system passes a partial oxidizer, a syngas cooler, a water-gas shift reactor, and an absorption system for separating carbon dioxide from the gaseous fuel, whereby said carbon dioxide is then dried and compressed before being sequestered.
On an industrial scale, aqueous solutions of organic bases, for example alkanolamines, are frequently used as absorbing fluids to remove carbon dioxide from fluid streams. When carbon dioxide dissolves, ionic products form from the base and the carbon dioxide. The absorbing fluid can be regenerated by expansion to a lower pressure, or stripping, with the ionic products back-reacting to liberate the carbon dioxide and/or the carbon dioxide being stripped off by steam. The absorbing fluid can be reused after the regeneration process.
However, in spite of the fact that high-pressure fluid is treated, the carbon dioxide separated from the fluid by a conventionally employed process has a low pressure close to atmospheric pressure. This is disadvantageous in that, for the above-described purpose of permanent storage the carbon dioxide must be pressurized from a low pressure to a pressure of about 150 bar (absolute pressure) which is required for injection. Carbon dioxide at a high pressure is also required for certain industrial uses, e.g., in the production of urea.
In the treatment of a high-pressure gaseous feed stream, a two-stage method is typically employed. A relatively small part of the regenerated absorption liquid (lean solvent) is fed in at the top of the absorber and a relatively large part of only partially regenerated absorption liquid (semi-lean solvent) is fed into the center of the absorber. The majority of the carbon dioxide is removed in the circuit of the partially regenerated absorption liquid (semi-lean loop) and only the polishing is performed using the regenerated absorption liquid. The regeneration step typically comprises expansion or flashing the carbon dioxide-rich absorbing fluid from the high pressure prevailing in the absorber to a lower pressure whereby the loaded absorption liquid is partially regenerated. A smaller part of the absorption liquid is thermally regenerated by direct or indirect heating.
The prior art discloses several processes in which the carbon dioxide is recovered at a pressure higher than atmospheric pressure. An advantage of carrying out the regeneration step at above atmospheric pressure is that low pressure stages of compression may be eliminated.
Thus, EP-A 768 365 teaches a process for removal of highly concentrated carbon dioxide from high-pressure natural gas which comprises an absorption step of bringing natural gas having a pressure of 30 kg/cm2 (30 bar absolute pressure) or greater into gas-liquid contact with an absorbing fluid; and a regeneration step of heating the carbon dioxide-rich absorbing fluid without depressurizing it, whereby high-pressure carbon dioxide is liberated.
U.S. Pat. No. 6,497,852 describes a carbon dioxide recovery process by preferentially absorbing carbon dioxide from a feed stream into a liquid absorbent fluid, pressurizing the resulting stream to a pressure sufficient to enable the stream to reach the top of a stripper at a pressure of 35 psia (2.4 bar absolute pressure) or greater, and stripping carbon dioxide from the stream in the stripper at a pressure of 35 psia (2.4 bar absolute pressure) or greater.
WO 2005/009592 relates to an acid gas regeneration process which is conducted under a pressure that exceeds 50 psia (3.5 bar absolute pressure) and does not exceed 300 psia (20 bar absolute pressure). The separated gas stream emerging from the regenerator is compressed and injected into a subsurface reservoir.
These processes wherein carbon dioxide is recovered at a pressure higher than atmospheric pressure, however, involve a significantly higher reboiler duty than the above mentioned two-stage method.